1. Field of the Invention
The invention relates to electrical quantity measurement, and more particularly, to such measurement of relatively small variations in the environment of a relatively large common-mode voltage.
2. Description of the Prior Art
The problem of measuring or coupling out a low-level signal across a high potential difference is an old problem which has been approached in various ways in the prior art. Basically, it is always necessary to use an isolating signal coupling of one type or another.
A typical example of the situation in which prior art instrumentations have been applied (and to which the present invention is also applicable) arises when current in a conductor must be monitored, but where the conductor itself is floating at some relatively high and possibly variable voltage. This floating voltage is sometimes referred to as a common-mode voltage. Among the techniques which have been used in the prior art to effectively reject these common-mode voltages while measuring the floating parameter, include the magnetically coupled current transformer, the differential amplifier, and various systems which convert the floating parameter to be measured into another form of radiatable or conductable signal which can be monitored at a remote point.
Probably the simplest device for the purposes, and one which is especially adapted to current measurement in the environment of at least moderately high common-mode voltage, is the current transformer having a core of magnetic material (frequently of torroidal form) through which the conductor carrying the current to be monitored passes, in effect as a single turn primary. A secondary winding on the magnetic core then produces a signal which is a function of the varying current to be monitored. The transformer primary-to-secondary insulation frequently including an air-gap around the high voltage conductor, provides the need isolation. Unfortunately, the inherently limited low frequency response of this type of transformer coupling severely limits its utility for wide current pulses or where the DC current componet (steady state value) must be included in the measurement.
The differential amplifier device has the inherent capability of amplifying the difference between two inputs while ignoring the actual common-mode voltage relative to ground. Typically, such a differential amplifier might have as its inputs the two ends of a meter shunt or standard resistance element inserted in series with the aforementioned conductor for current monitoring. Where the common-mode voltage is relatively low (for example 50 volts or less) or where the signal differential across the sensing element is relatively large, the differential amplifier technique can often be used with satisfactory results. Otherwise, it must be expecially designed for the application, and even then, practical differential amplifiers have limited accuracy due to drift and zeroing errors which are especially troublesome where the small signals must be "sorted out" from a relatively high common-mode voltage. If the common-mode voltage is quite high (hundreds or thousands of volts) then amplifier insulation problems must also be dealt with.
A relatively new device for isolated signal transmission is available as a subassembly and is used in the combination of the present invention. This device is the so-called optically coupled isolator (OCI). In its basic form, a light-emitting diode or similar electric-to-light transducer provides a light source of intensity porportional to the signal being monitored and this output is coupled to a photosensitive device via an optical path, frequently in the form of a fiber optic bundle of at least one optical fiber. Since the optical path is inherently an insulating medium of very high resistivity, such a device has a capability for isolating the large common-mode voltage. Insulation resistances on the order of 10.sup.12 ohms or more are readily achieved and very low capacitances are exhibited (one picofarad or less). Those values of resistance and capacitance are easily realized at common-mode voltages on the order of 2500 volts dc with transients of 1000 volts per microsecond.
A commercially available OCI includes the light-emitting diode and photo transistor or photo diode with amplifying transistor coupled thereto at input and output ends, respectively, of an optical fiber link. Such a commercial device is available, as the HP 5082-4354, 4354, manufactured by Hewlett Packard Company of Palo Alto, Calif., for example.
The OCI per se, however, does have an important disadvantage used in its obvious form, this being the inherent non-linearity of the semiconductor elements used i.e., the LED and photo-transistor or photodiode elements which act as input and output transducers, respectively, for the length of optical fiber.
If the application requires a highly linear relationship between input and output, it is necessary to severely limit the dynamic range over which the unit operates. This inevitably leads to very low output levels and complex networks for the compensation of thermal and other effects which might otherwise mask the desired measurement.
The manner in which the present invention employs the OCI to obtain its inherent advantages of frequency response extending down to dc, and the very high order of insulation against the common-mode voltage, will be understood as this description proceeds.